Present board assembly processes using reflow soldering require the use of solder paste to provide solder necessary to form solder joints between components and the printed circuit board. This requires that solder paste be applied using either printing with a solder stencil or screen, or by dispensing the paste with a syringe. Syringe application is very time consuming, while screen printing requires use of a stencil which is unique to each circuit board and requires setup with associated delays in production cycle time and throughput. The use of solder paste requires precise control of the chemistry and rheology of the paste in order to insure that a material with both acceptable stencil printing and soldering characteristics is provided. For example, to insure acceptable printing characteristics, a liquid vehicle and binding agents must be added, constituting about 50% by volume of the paste. The binding agents are generally some form of resin which also assists in the soldering process. These resins remain behind after solder reflow and must be removed from the board by washing with ozone-depleting chloroflorocarbon solvents to allow probe testing of the boards after reflow and to insure reliability. In wave soldering, it has been possible to reduce the solids in the flux so that the small amount of residues remaining after soldering do not interfere with testing. Because of the liquid vehicle requirements for solder paste, this has so far been impossible to achieve for reflow processes.
Because of the complex makeup of the solder paste, it is, of necessity, an expensive material, and a significant cost advantage could be accrued by eliminating it from a process. One method of eliminating solder paste is to provide printed circuit boards with a precoating of solder covering the underlying copper conductors. Methods of providing a thick solder coating using plating methods have been documented in the literature. For example, "Fluxless SMD Soldering" by Hendricks and Inpyn of General Electric Company in Circuits Manufacturing, October 1984, details the use of heavy solder plating to assemble printed circuit boards and leadless components. Other methods of providing solder plating, such as dipping, have been used, but results in coatings that are thin in some areas and thick in other areas, allowing an intermetallic compound of copper and tin to be exposed to the oxidizing environment of the atmosphere. When this occurs, the board becomes unsolderable, defeating the intention of the solder coating. In addition, dipping results in the plated through holes of the circuit board becoming filled with solder, preventing the insertion of leaded components into the holes.
By limiting the use of flux or vehicle in solder paste and wave solder operations, the board assembly process becomes simplified and reduces materials, labor and cycle time and increases quality. Elimination of flux and vehicle also eliminate the need to clean the circuit board after soldering. In addition, elimination of solder paste and wave soldering also would reduce the incidence of solder balls and solder shorting resulting from the reflow or wave soldering operations.
When boards are plated with heavy coatings of solder, they must still be reflowed in order to insure adequate alloying of the solder to the underlying copper. This reflowing procedure is known as fusing, and when heavy coatings of solder are fused, the solder wets and flows into the plated through holes thereby blocking the plated through holes and preventing insertion of leaded components.
Clearly, these methods of eliminating solder paste or wave soldering are suitable for use in surface mounted components, where all components are placed on the surface of the printed circuit board. However, all of the previous methods result in filling the plated through hole with a solder plug, making it impossible to insert leaded components into the holes. An improved method of assembling printed circuit assemblies that eliminates solder paste and provides a means to utilize leaded components is certainly desirable.